A. Field of Invention
The invention relates to the field of performance enhancing shoe components and methods for making the same. More specifically, this invention relates to the field of performance enhancing shoe components which provide energy return and improve athletic performance, and methods of making the same.
B. Description of the Related Art
There is a high demand for athletic equipment which enhances the performance of athletes. As athletic events become more competitive, small improvements in performance become increasingly important. Athletic equipment which enhances the performance of athletes by only a few tenths or hundredths of a second, or a fraction of a centimeter, may provide the "winning edge" sought by athletes. For this reason, athletes are constantly seeking equipment which will improve their performance. The designs of all types of equipment from bicycles to tennis rackets are constantly scrutinized for features which may improve performance. For example, racing bicycles have been radically redesigned to improve performance while reducing weight and friction. Cycling helmets have also been redesigned to reduce weight and wind resistance.
This high demand for performance enhancing athletic equipment includes the art of athletic shoes and shoe components. Various features have been included in shoes for promotional reasons or for comfort. Most improvements in athletic shoes have been made in the physical and structural composition of the interior and exterior elements of soles of athletic shoes to improve the comfort and appearance of the shoes. While it has long been desired to produce athletic shoes which provide meaningful increases in athletic performance, few, if any, changes in athletic shoes have produced measurable improvements in performance. Often an improvement in athletic shoe performance is sought but is not attained because the design of the resulting shoe becomes a compromise between comfort and cost. No prior shoe features provide substantial energy return to the user. Similarly, prior shoe features did not provide energy return in the form of lift, spring or bounce.
The material most frequently used for insoles in athletic shoes is ethylene vinyl acetate ("EVA"). EVA is a lightweight, inexpensive, sponge-like material which does not provide significant energy return. Examples of shoe insoles made from EVA are disclosed in a U.S. patent to Fujita et al. (U.S. Pat. No. 4,418,483), which is incorporated herein by reference. This patent teaches making shoe midsoles by combining EVA with a styrene-butadiene rubber. While such midsoles have excellent cushioning properties, they do not enhance athletic performance by providing energy return to the user. Rather, the cushioning effect of the EVA material reduces athletic performance by absorbing energy.
Fujita et al. also disclose combining 1,4-polybutadiene with other rubbers, such a 1,2-polybutadiene, to make hard shoe outsoles or shoe midsoles. These other rubbers do not provide substantial energy return, and these rubbers reduce the energy return characteristics of the 1,4-polybutadiene. For example, in its cured state, 1,2-polybutadiene is a hard material which lacks the ability to be compressed and rebound to provide energy return. In its uncured state, 1,2-polybutadiene (syndiotactic) may exhibit mild elastomeric properties. When high energy return rubbers such as 1,4-polybutadiene rubber are combined with 1,2-polybutadiene, the 1,2-polybutadiene typically controls the characteristics of the resulting composition and reduces its energy return. Similarly, Fujita et al. teach the inclusion of fillers such as hard clay and silica, which further reduce the energy return of the shoe component.
A U.S. patent to S. P. Chang and R. I. Chang (U.S. Pat. No. 5,147,589), which is incorporated herein by reference, discloses a method of making shoe soles of a polymer blend which is a mixture of rubbers including thermoplastic elastomers, styrene-butadiene rubber, acrylonitrile-butadiene rubber, and plastics such as polystyrene, EVA, or polyvinyl chloride. When high energy return rubbers such as 1,4-polybutadiene rubber are incorporated into such shoe soles, the high energy rubbers compose only a small weight percentage of the total rubber in the shoe, and, therefore, the shoes do not provide substantial energy return. Chang et al. also teach the inclusion of non-energy returning materials ("fillers"), including white hard clay, white carbon VN-3 filler, carbonates and silica. These materials will modify some characteristics, such as durability or strength, of the shoe part, but will also decrease energy return.
A U.S. patent to T. Noyama and A. Nakahara (U.S. Pat. No. 5,288,446) discloses a process for imparting strength to rubber articles. This patent is incorporated by reference herein. Noyama et al. teach enhanced strength by adding fillers such as metal salts of alpha, beta-unsaturated fatty acids, nylon and calcium carbonate. Such fillers harden the rubber article and improve its strength at the expense of energy return. Because such hardened compositions are less compressible, they can injure the user.
A U.S. patent to Sullivan et al. U.S. Pat. No. 4,910,886) discloses a shoe insert for total shock absorption, but not for energy return. This patent is incorporated by reference herein. The innersole of Sullivan et al. contains predominant amounts of a substantially non-deformable, expandable polymer, such as elastomeric urethane. Such materials are not known for providing energy return or for improving athletic performance.
While the energy return of 1,4-polybutadiene, in the form of super balls, for example, is known, those of ordinary skill in the shoe art have not looked to shoe components made of 1,4-polybutadiene and other high energy return rubbers to improve athletic performance. The makers of athletic shoes have failed to appreciate the significant energy return available from shoe components of 1,4-polybutadiene in combination with other high energy return rubbers such as natural rubber, synthetic isoprene rubber, polyisoprene, butadiene acrylonitrile rubber or ethylenepropylene diene modified rubber.